EFFECT OF LOW AND HIGH TEMPERATURES 293 



well as the comparative slowness of the operation, have 

 prevented their general adoption. Germ-free milk for 

 laboratory purposes can be obtained by filtration through 

 porcelain filters, but much of the fat is removed in the 

 process as well as the bacteria. 



(ii) The Effect of Low and High Temperatures, (a) 

 Cooling. One of the most effective means of prevent- 

 ing milk from undergoing fermentative changes is to 

 cool it and keep it at a low temperature. The sooner 

 this can be done after the milk is drawn from the 

 cow the better. Most of the bacteria which damage 

 milk, from the consumer's point of view, grow very 

 slowly at 10 C. (50 F.), and where it can be cooled 

 immediately after milking, and maintained at this or a 

 lower temperature, it will keep without any recognizable 

 change for several days. Moreover, cooling does not 

 appreciably alter the flavour of the product. 



It must be noted that the bacteria are not killed by 

 this treatment, nor can they be killed by the application 

 of the lowest temperatures attainable : they are merely 

 checked for the time, and will develop readily- enough 

 when the temperature is raised. 



(3) Sterilization. The sterilization of milk can be 

 secured by heating it under pressure to a temperature 

 of 1 10 C. or 1 1 5 C. (230 to 240 F.) for half an hour. 

 Boiling it for. twenty minutes destroys all the common 

 forms, but the spores of certain species which are con- 

 veyed into milk by particles of hay and soil resist the 

 treatment for some time, and may ultimately grow and 

 set up objectionable chemical changes in it when the 

 temperature is reduced. However, continuous boiling 

 for an hour or .two will destroy the spores of all bacteria, 

 and render it sterile. 



